Electrolytic production of alkanesulfonic acids



Patented Sept. 5, 1950 ELECTROLYTIC PRODUCTION OF ALKANESULFONIC ACIDS Bruce H. Brown, Skokie, Ill., assignor to Standard Oil Company, Chicago, 111., a corporation of Indiana No Drawing. Application June 26, 1946, Serial No. 679,563

10 Claims. 1

This invention relates to an electrolytic proc ess for the oxidation of alkyl disulfides. More particularly it relates to a process for the preparation of alkanesulfcnic acids by the anodic oxidation of sulfonic acid solutions of alkyl disulfides.

Although numerous chemical methods have been proposed for the oxidation of organic sulfur compounds such as alkyl disulfides to sulfonic acids, it has been found that they irequently result in over-oxidation to produce sulfuric acid or yield considerable quantities of lower oxidation products such as sulfoxides and sulfones together with the desired sulfonic acids. Moreover, said chemical methods entail costly equipment and are not readily adaptable to continuous processing. I have discovered a smooth electrolytic method for the oxidation of alkyl disulfides to produce sulfonic acids in good yields.

One object of my invention is to provide an electrolytic process for the oxidation of alkyl disulfides. Another object is to provide an electrolytic process for the oxidation of alkyl disulfides, in which process a sulfonic acid. or a mixture of sulfonic acids is employed as a solvent for the alkyl disulfide. Another object of my invention is to provide an electrolytic process for the oxidation of alkyl disulfides wherein the initial electrolyte consists essentially of a sulionic acid. These and other objects of my invention will become apparent from the ensuing description thereof.

' It has been observed by Fichter and Wenk that diphenyl disulfide can be oxidized to benzenesulfonic acid at a platinum anode in an electrolyte consisting of glacial acetic acid and concentrated hydrochloric acid (Ber. 45, 1376-1383 (1912)). However, these investigators point out (100. cit., page 1383) that when dibenzyl disulfide was electrolytically oxidized, benzyl disulfoxide was obtained rather than benzylsulfonic acid. It

was surprising, therefore, to find that when sulionic acids are employed as solvents for alkyl disulfides, th resultant solutions can be smoothly oxidized. to yield alkanesulfonic acids at the anode of an electrolytic cell.

The term alkyl disulfide as used in this specification and the appended claims is intended to denote a disulfide wherein at least one sulfur atom is linked directly to the carbon atom of an aliphatic chain.

Suitable dialkyl disulfide ieedstocks include dimethyl, methyl ethyl, diethyl, methyl propyl, dipropyl, dicyclohexyl, dicetyl, and dilauryl di- The feed stock may be a pure compound or a mixture of compounds, optionally in admixture with substances, such as parafiin hydrocarbons which are not normally oxidized to an appreciable extent under the conditions employed in my process. I prefer to employ dialkyl disulfide charging stocks, particularly those containing no substituents in the alkyl groups. Moreover, it is highly desirable to use symmete rical dialkyl disulfides, viz., those containing two identical alkyl groups, since they yield only one sulfonic acid upon electrolytic oxidation.

A suitable feed stock is a mixture consisting essentially of dialkyl disulfides of the type or tained as a lay-product in the extraction of mercaptans from petroleum distillates with a causticsolutizer solution followed by regeneration of the solution-in the presence of an oxidation catalyst to produce a mixture of dialkyl disulfides which are separated from the caustic solution by settling or by extraction with a solvent, such as petroleum hexane, naphtha, or the like. Either the crude disulfides or a fraction thereof can be oxidized by my technique.

The extraction of petroleum distillate with caustic-solutizer solutions has been described by D. L. Yabroff and E. R. White (Ind. Eng. Chem. 32, 959-953 (1940)) and has been reviewed by V. A. Kalichevsky and B. A. Stagner (Chemical Refining of Petroleum-revised edition 194 .3, pp. 218-220). The production of disulfides by catalytic oxidation of mercaptans (present as mercap-tides) in caustic-solutizer solutions has been described by J. P. ODonnell (The Oil and Gas Journal, pp. 45-47 (July 1, 1944) The sulfonic acid employed as the solvent and initial electrolyte'in the process of my inven tion can be the same as or different from the sulfonic acid or acids produced by the electrolytic oxidation of the alkyl disulfide feed stock. However, in the interests of avoiding possible difficultiss in separating the sulfonic acid produced in my process from the sulfonic acid employed as the solvent, it is expedient to employ as the solvent a sulfonic acid identical with the one produced in th anodic oxidation process. The employment of aromatic sulfonic acids as solvents and electrolytes in my process is not excluded, as methods are known for separating aromatic sulfonic acids from alkanesulfonic acids and these may be applied to separate the arc- :natic sulfonic acid solvent from the alkanesulionic acids produced by the electrolytic oxidation of a dialkyl disulfide or the electrolytic oxidation of an alkyl aryl disulfide.

The concentration of alkyl disulfide in the sulfonic acid solvent may vary from about 1 percent by weight up to the limit of solubility of the disulfide in the sulfonic acid. Ordinarily I employ solutions containing about 15 weight percent of disulfide or even less, for example about weight percent; The water content of the sulfonic acid employed as the solvent and initial electrolyte in the present process can be varied over a wide range, for example between about 1 and about 25 weight percent, based. on the weight of said sulfonic acid, e. g., about 'weight'percent; However, since it is commercially desirable to-produce alkanesulfonic acids containing as little water as possible, it is preferablecthat'the solution being electrolyzed contain not more than about 10 weight percent of water initially The electrolytic oxidation of sulfonic acid solutions of alkyl disulfides may be effected by means of direct current, alternating current or mixedcurrent. Practical, although not necessarily limiting, voltagesfor' my processrange from. about 1 to about 10 volts; I prefer to: avoid relatively high: current densities above about 0.02' ampere per square cm., when direct: current is employed,

especially for the: electrolytic oxidation of lower dialkyl'disulfides containing not more than about 4 carbon atoms in the:- molecule. In the electrolytic oxidation. of these lower dialkyl' disulfides it has been observed that theuse of alternating current of low current densities below about 0.1 amp. per squarecmresults in smooth. conversion of the feed stock to. alkanesulfonic' acid accompanied by Y relatively little over-oxidation to sulfuric acid, as

will be brought out more fully in the examples below.

Electrolytic oxidation of alkyl disulfides in acc'ordance'with this invention has been efiected at about room temperature, but temperature is not a critical variable in this process and can be varied over a considerable rangeboth below and above room temperature, for example, between about 10F; and about 170 F. Desirable temperatures fall within the rangeof about 90'to about 150 F.

Alkanesulfonic acids, especially in aqueous solutions, are corrosive materials and care should accordingly be exercised in the choice of materials used forthe construction of the electrolytic cell employed for their production, as well as for auxiliary equipment, such as feed and discharge conduits, separating vessels, fractionating and storage equipment, etc. Equipment for handling alkanesulfonic acids can be made of or lined with glass or other resistant ceramic materials, structural carbon, stainless steels or noble metals such as gold, silver and the like;

Platinum electrodes have been employed in practicing the process of my invention, but other electrode materials may be employed, e. g., carbon, gold, or other conductors which suffer no appreciable attack'by the sulfonic' acids under the electrolysis conditions.

The following examples are intended to illustrate but not to limit my invention.

Example 1 The electrolytic oxidation cell consisted of a platinum vessel serving both as a container for the electrolyte and as the anode and, suspended therein, an alumina diaphragm containing a platinum cathode. The electrolyte was a mixture of 85 weight percent of alkanesulfonic acids containing between '1 and 4' carbon atoms in the alkyl group (average molecular weight about 115) and 15 weight percent of water. This electrolyte was saturated withdimethyl: disulfidetoproduce a solution wherein the solute concentration was approximately 5 weight percent. This solution was anodically oxidized with direct current for 20 hours at the ambient temperature, employing an average of 9 volts and 0.1 amp. As a result, methane-sulfonic acid. was formed-,, as measured by titration by aqueous caustic; at a cell efiiciency of 37.8 percent, based on the acid only.

Example 2 The cathode of the cell employed in Example 1 was replaced by a larger one. The solvent employed in Example 1 was saturated with diethyl disulfide to yield a solution thereof having a solute concentration of approximately 10 weight percent. This solution was subjected to electrolysis with direct current for 24 hours at room temperature, 6 volts and 0.29 amp. to produce ethanesulfonic acid at a cell eificiency of 58.6 percent. No undesirable effects, such as reduction of diethyl disulfide to ethylmercaptan, were observed during the electrolysis.

Examples. 3 and. 4

Example 3 4 Type of current D. C Electrolyte acid Ethm1esullonic Dlsullldo feed diethyl Current density, amps /cm 2 Feradays, totaL Voltage, average Sulfonic acid yield. Sulfuric acid yield l Moles Liz-S04 per Faraday. Ratio sullonic/sulfuric acid" Current efficiency A. C. Ethancsulfonic. diethyl.

Although the above examples illustrate batch operations, my invention is not thus limited, since the electrolytic oxidation cells can be designed and arranged for continuous operation withcontinuous or intermittent introduction of asulfonic acid solution of alkyl disulfide to one or a series of cells and continuous or intermittent withdrawal of sulfonic acid-enriched electrolyte from the cell or from one of the series of cells. When the electrolytic oxidation is effected in a series of cells with passage of partially oxidized solutions of alkyldisulfides from cell to cell in the series, dififerent operating conditions maybe employed in different cells in order to obtain maximum current efiiciency.

The sulfonic acid may be recovered as pure acid by adjusting cell conditionsand feed proportions so that all the water and disulfide are used up in the electrolysis, the cell effluent then being substantially pure sulfonic acid.

It is contemplated that the addition of small proportions (about 0.001 to about 1 weightpercent) of oxidation promoters to the celL electrolyte may be beneficial, for example promoters such as ceric sulphate, vanadium sulphate and the like.

I claim:

1. A process for the production of an aikanesulfonic acid which comprises subjecting a solution of a dialkyl disulfide having 1 to 4 carbon atoms, inclusive, in the alkyl group in an electrolyte consisting [essentially of an alkanesuiionic acid containing between about 1 and about 25 weight percent by weight of water to direct current electrolysis at a temperature between, about 10 F. and about 170 F., at a voltage between about 1 and about 10 volts and at 9. current density not exceeding about 0.02 ampere.

per square centimeter.

2. The process of claim 1 wherein the disul-" tide is diethyl disulfide.

3. The processot claim 1 wherein the disulflde is dimethyl disul fide. I

4. A process for the production of an alkanesulfonic acid which comprises electrolyzing a solution of an Jallryl disulfide in an electrolyte. consisting essentially of a sulfonic acid containing between about 1 and about 25 percent by weight of water at an oxidation temperature between about 10 F. and about 170 F. and at a voltage between about 1 and about 10 volts.

5. The process of claim 4 which is conducted with direct current.

-6. The processor claim 4 which is conducted with alternating current.

7. The process of claim 4 wherein the alkyl disulflde contains 1 to 4 carbon atoms, inclusive, per alkyl group.

6 8. A process for the production of an alkanesulfonic acid which comprises electrolyzing a solution of an alkyl disulfide containing 1 to 4 carbon atoms, inclusive, per alkyl group in an electrolyte consisting essentially of a sulfonic acid containing between about 1 and about 25 percent by weight of water by means of alterhating current at a temperature between about 10 F. and about F., at a voltage between about 1 and about 10 volts and at a current density not exceeding about 0.1 ampere per square centimeter.

9. The process of claim 8 wherein the disulfide is diethyl disulfide.

10. The process of claim 8 wherein the disulfide is dimethyl disulfide.

BRUCE K. BROWN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 729,502 Moest May 26, 1903 2,140,194 Yabrofi et a1. Dec. 13, 1938 OTHER REFERENCES Transactions of the Electrochemical Society, vol. 65, (1934), pp. 307-308.

Deutsche Chemische Gesellschaft,Berichte,vol. 45. (1912), p. 1383. 

4. A PROCESS FOR THE PRODUCTION OF AN ALKANESULFONIC ACID WHICH COMPRISES ELECTROLYZING A SOLUTION OF AN ALKYL DISULFIDE IN AN ELECTROLYTE CONSISTING ESSENTIALLY OF A SULFONIC ACID CONTAINING BETWEEN ABOUT 1 AND ABOUT 25 PERCENT BY WEIGHT OF WATER AT AN OXIDATION TEMPERATURE BETWEEN ABOUT 10* F. AND ABOUT 170*F. AND AT A VOLTAGE BETWEEN ABOUT 1 AND ABOUT 10 VOLTS. 